In 2004, in the US, countless members of the Democratic Party fell in love. The object of their affection was a young man with a beautiful face, large eyes and a winning smile. His name was John Edwards. Though he had been in national politics barely five years and had no accomplishments to speak of, he went on to become the Democratic Party nominee for vice president.

What accounted for his sudden success? The cheeky answer would be that Americans are superficial and judge politicians by their looks, which in this case proved palpably misguided. As you may recall, while Edwards was celebrated for his marriage to a down-to-earth, middle-aged lawyer, he was cheating on the side with a younger beauty whom he had put on his campaign payroll.

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Studies in the US do indeed suggest that American voters frequently choose candidates on their looks. That seems to be the implication of a 2006 study involving Harvard students who were asked to evaluate politicians running for governor strictly on the basis of a silent ten-second video clip. The experiment, conducted by Daniel J Benjamin and Jesse M Shapiro, showed astounding results. Fifty-eight per cent of the students picked the candidate who would go on to win. This suggests that many voters are doing exactly what these students were: judging the candidates by their looks.

In a second study published a year later by Alexander Todorov, this time involving Princeton students, the results were even starker. Rather than being shown video clips, the students were briefly exposed to candidates' headshots. Once again the students showed no reluctance to draw inferences about politicians from their image alone. And once again they showed an uncanny ability to pick the candidates who went on to win. (They chose those who looked the most competent.)

But it is not just Americans who let a candidate's appearance affect their vote. Social scientists say this is common around the world. A study of British voters in non-partisan community elections found that they overwhelmingly went for the candidates whose pictures neutral observers rated trustworthy, empathic and competent.

Worse, when British students at the University of Liverpool were asked to select candidates on the basis of the shape of their faces alone so the identity of the candidates - George W Bush and John Kerry - could not be determined, they chose Bush just as American voters had. Why? It was apparently because Bush had a more masculine face. The researchers found that voters, whether British or American, prefer politicians with classic masculine features in wartime (the study was conducted at the height of the Iraq War) and classic feminine features in peacetime. (The study, "Facial Appearance Affects Voting Decisions", wasn't a hit job on the British by Americans. It was conducted by UK researchers.)

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Why do humans draw inferences from such obviously superficial criteria as an individual's facial contours? Evolutionary psychologists say the answer is that during the Stone Age, the two-and-a-half-million year-long period when the human brain mainly evolved, lightning -quick judgements often had to be made about strangers. Nature rewarded those who could estimate at a glance whether someone else was competent, empathic or a threat. Those who proved good at the task were more likely to survive and pass along their genes, enhancing their fitness, as evolutionary scientists put it.

Just how quickly do we make these judgements? In one 2005 study, "Inferences of Competence from Faces Predict Election Outcomes", published by the American Association for the Advancement of Science, it was established that our snap judgements about people take shape within 167 milliseconds of seeing them - that's faster than it takes to blink. When subjects in experiments are given more time to make a decision, they use it merely to become more confident that their initial impression was correct. They don't rethink their decision. All of this takes place largely outside of conscious awareness. In other words, your brain makes up its mind about someone you meet in less time than it would take you to shake their hand.

Knowledgeable voters base their decisions on more than the snap judgements that their brain makes. But most voters aren't knowledgeable. You can keep that in mind the next time an election goes against your candidate. You can legitimately comfort yourself with the thought that the voters knew not what they were doing.

Rick Shenkman is the publisher of the History News Network and the author of Political Animals: How Our Stone-Age Brain Gets in the Way of Smart Politics (Basic Books)